The nonequilibrium evolution through the QCD critical region may influence the critical observables, such as skewness and kurtosis of the order parameter [1]}, [2]}, [3]}, [4]}. Skewness and kurtosis of nonequilibrium evolution differ in magnitude or in sign from equilibrium expectations. Taking the skewness of net proton for an example, the equilibrium expectations are positive at the freeze-out point [5]}, [6]}, [7]}. The measured values by STAR experiment are always negative, i.e. below the Poisson baseline [8]}. The negative value may be caused by nonequilibrium effect. Therefore, the discovery of the QCD critical point with a solid experimental basis is highly challenging and has attracted a large number of theoretical and experimental interests.
